plat/arm: Remove ARM_LINUX_KERNEL_AS_BL33 relying on RESET_TO_BL31

So far the ARM platform Makefile would require that RESET_TO_BL31 is set
when we ask for the ARM_LINUX_KERNEL_AS_BL33 feature.
There is no real technical reason for that, and the one place in the
code where this was needed has been fixed.

Remove the requirement of those two options to be always enabled
together.
This enables the direct kernel boot feature for the Foundation FVP
(as described in the documentation), which requires a BL1/FIP
combination to boot, so cannot use RESET_TO_BL31.

Change-Id: I6814797b6431b6614d684bab3c5830bfd9481851
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

plat/arm/common/arm_common.mk

@@ -86,11 +86,7 @@ $(eval $(call assert_boolean,ARM_LINUX_KERNEL_AS_BL33))
 $(eval $(call add_define,ARM_LINUX_KERNEL_AS_BL33))
 
 ifeq (${ARM_LINUX_KERNEL_AS_BL33},1)
-  ifeq (${ARCH},aarch64)
-    ifneq (${RESET_TO_BL31},1)
-      $(error "ARM_LINUX_KERNEL_AS_BL33 is only available if RESET_TO_BL31=1.")
-    endif
-  else
+  ifneq (${ARCH},aarch64)
     ifneq (${RESET_TO_SP_MIN},1)
       $(error "ARM_LINUX_KERNEL_AS_BL33 is only available if RESET_TO_SP_MIN=1.")
     endif